Item 31: Use bounded wildcards to increase API flexibility

Using wildcard types in your APIs, while tricky, makes the APIs far more flexible. If you write a library that will be widely used, the proper use of wildcard types should be considered mandatory. Remember the basic rule: producer-extends, consumer-super (PECS). Also remember that all Comparables and Comparators are consumers.

Example:

void process(Consumer<Number> consumer);

void process(Consumer<? super Number> consumer);

Likewise, type parameters in covariant position:

T produce(Producer<T> p);

T produce(Producer<? extends T> p);

Use the first checkbox to ignore invariant classes. Invariant classes (e.g. java.util.List<T>) are classes that both accept values (via its List.add(T) method) and produce values (via its T List.get() method). On the other hand, contravariant classes only receive values, e.g. java.util.function.Consumer<T> with the only method accept(T), and covariant classes only produce values, e.g. java.util.function.Supplier<T> with the only method T get(). People are often OK with bounded wildcards in covariant/contravariant classes but afraid of wildcards in invariant classes, e.g. void process(List<? extends T> l). Turn this setting off to ignore these invariant classes and leave them rigidly typed, e.g. void process(List<T> l).

Uncheck the second checkbox to ignore private methods, which can be considered as not a part of the public API.

Uncheck the third checkbox to ignore instance methods and only report static methods.